The present invention relates to a lightguide including a plastic tube, especially a tube made from carbon fluorine resin like Teflon.RTM. FEP, filled with light conducting liquid and seated on both ends by windows, made from cylindrically formed transparent plugs of glass.
A light conducting liquid for the visible spectral range must have the following properties.
a) High transparency in the spectral range between 400 and 700 nanometers, up to a length of the lightguide of 5-10 meters. PA0 b) Photochemical stability, especially for radiation delivered by light sources being frequently used in the visible range, like tungsten halogen Incandescent lamps as for example a 150 Watt projector lamp with a cold light reflector. PA0 c) The liquid must be physiologically safe. Since liquid lightguldes are also used in the medical endoscopy, the liquid must not be toxic o PA0 d) The liquid should not be easily combustible or flammable. PA0 e) The freezing point should not be higher than -20.degree. C. because the liqhtguide must also be able to be shipped during winter time. In addition the lightguide should be tunctioning also at environmental temperatures below freezing. PA0 f) The light guiding liquid must be compatible with the material of the cladding tube. That means that the thermal expansion coefficients of liquid and tubing may not differ significantly, so that no bubbles appear in the liquid of the lightguide when the environmental temperature fluctuates between -20.degree. C. and +50.degree. C. The liquid must not attack or swell the material of the cloladding tube which consists in most cases of a carbon fluorine polymer or copolymer. I his must also apply to cases where the tube is internally lined with a thin layer of Teflon.RTM. AF. PA0 g) The lightguldes whose major application is In the field of medical endoscopy should not be affected by sterilization processes, using ethylene oxide (ETO) as the sterilizing gas. PA0 h) A high refractive index of the light conducting liquid is indispensable to ensure for total reflection of the radiation at the inner surface of the cladding tube, to obtain a high numerical aperture of the lightguide and as a consequence a greatest possible independence of transmission from bonding of the lightguide.
The liquid also must not diffuse through the wall of the tubing having a thickness of typically 0.3-0.5 mm, for a time period Of Lip to several years. PA1 To guarantee this, the liquid must have a high boiling point or/and it should be hygrosoopio.
U.S. Pat. No. 4,009,382 describes a liquid lightguide with a liquid conisisting of an aqueous inorganic salt solution, In gonoral a calcium chloride solution With a high concentration resulting In a refractive index of n=1,435. A higher value for the refractive index would be desirable, however, a higher concentration of the salt solution would result in precipilation of salt at a temperature only a few degrees below room temperature, thus making the light guide opaque. The calcium chloride solution is hygroscopic, but the optical transmission of the corresponding liquid lightguide in the red spectral range Is insufficient, due to the high concentration of OH groups in the aqueous salt solution.
There are other liquid lightguides known from DE 3644839.7-51 where the liquid consists essentially of di-, tri-or tetra-ethylene glycol with a certain amount of water. These lightguides, too, do not allow for a higher value of the refractive Index as n=1,445 for the liquid core.
In addition, the concentration of OH groups, though smaller than with the aqueous salt solution, is still at such a high level that the transmission in the red spectral range at lengths of a few meters for the lightguide is not satisfying.
Another liquid lightguide has become known from DE 19508752.6, where the liquid core consists of an aqueous calcium chloride solution using, however, mainly heavy water (D.sub.2 O) instead of light water (H.sub.2 O). This lightguide has excellent transmission In the red spectral range, but the value of the refractive index is limited to n&lt;1.430 and in addition, the cost of such a liquid is extremely high due to the high price for heavy water.